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Snow Melt Mavens, I need your advice

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RayWohlfarth
RayWohlfarth Member Posts: 1,781

I came across this project from a long time ago. It was a snow melt system. When we started the boiler it ran for an hour and a half with the bypass valve closed. There was little change in the water temperature. Ever impatient Ray partially closed the bypass valve and allowed the water temperature in the boiler to raise to 160 degrees F. The loop heated quickly in like a half hour. My question to you is: Did throttling the bypass valve actually shorten the loop heating time or was it just a coincidence. The boiler still provided the same Btus whether the valve was closed or not. I would love your thoughts.

snow melt system.jpg snow melt bypass 2.jpg
Ray Wohlfarth
Boiler Lessons

Comments

  • mattmia2
    mattmia2 Member Posts: 13,244

    Pressure from closing the bypass some opened a stuck thermostatic mixing valve?

  • Jamie Hall
    Jamie Hall Member Posts: 26,120

    Interesting, @RayWohlfarth . And, I think, more complicated than it looks…

    But the first question is… is there also a pump in the boiler, or is that one circulator the only one? If that one circulator is the only one, then a big part of the explanation is that with the bypass cracked open a part of the low from the pump was going through the bypass from the return to the feed, and only part of it going through the boiler. And also, if that is the only pump, then the BTUh power output to the slab will be very nearly the same in both cases — unless the boiler has a remarkably high head loos. With the bypass partly open, you are getting a 60 degree rise through the boiler, whereas with it closed you got a 20 degree rise — suggesting less flow though the boiler (that old hydronic equation just can't be fiddled!).

    I don't think it made a whole lot of difference in the loop heating time.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ratio
    ratio Member Posts: 4,039

    ISTR @DanHolohan talking about that very thing. When I read the explanation, it made perfect sense, but I've lost it since then.

  • mattmia2
    mattmia2 Member Posts: 13,244

    Wait if the bypass is completely open, why would any of the water want to go through the loop (or the boiler depending on where the circulator(s) is)? The bypass is less resistance to the circulator than the loop if the circulator is on the boiler. The bypass is less resistance than the boiler if the circulator is on the loop. I assume the boiler was cycling on high limit.

  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,638

    This was something that came up when the Dead Men began converting gravity systems to pumped systems. By bypassing most of the cool water returning from the system the boiler was able to come up to high temperature (and not condense), and that speeded up the heating out there in the system.

    Retired and loving it.
    Mad Dog_2
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,781

    @Jamie Hall It just had the one pump.

    @mattmia2 I didnt want to open the valve all the way. I didnt trust the low water cutoff.

    It was an old cast iron sectional boiler. It was a strange thing. Thanks everyone.

    @DanHolohan Thanks for still teaching me something new

    Ray Wohlfarth
    Boiler Lessons
  • hot_rod
    hot_rod Member Posts: 25,248

    what type of boiler? What was the SwT with the valve closed, and delta?

    Was the system under load ? Melting snow during the adjustment?

    With the bypass closed the boiler must have been seeing cool return? 30 degrees or lower?

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GGross
  • GroundUp
    GroundUp Member Posts: 2,361

    Is Ray absolutely certain that the change occurred after the valve was partially closed? If the valve began as closed and then was partially opened, this type of reaction makes perfect sense and is exactly what Dan described above. It's very common with low temp systems and non-condensing boilers to utilize a bypass this way and they behave completely opposite of what you describe here. Perhaps the ball in your valve was machined or installed incorrectly, where a perpendicular handle was open and a parallel handle was closed (I have seen this several times in my career)?

  • ScottSecor
    ScottSecor Member Posts: 992

    @DanHolohan I suspect many of those older gravity systems also had a tankless coil or side-arm style for dhw needs. Bypass might be more important on boilers that also provide dhw from a practical standpoint.

  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,638

    I agree, Scott. Thanks.

    Retired and loving it.
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,781

    @Hotrod It was a very old cast iron sectional boiler with an atmospheric burner. It was not melting snow. I wanted to see if the loop would heat, the owners complaint. It was just weird that this happened Not sure if it was due to the valve partially closed. I still think about this job 20 some years later

    @GroundUp I have the patience of a corgi and was looking at some way to get outta there. It ran with no significant temperature rise on the return until I partially closed the valve. Weird huh. It could have been coincidence

    Thanks

    Ray Wohlfarth
    Boiler Lessons
    PC7060
  • PC7060
    PC7060 Member Posts: 1,675
    edited June 7
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,781

    @PC7060 Thanks for that

    Ray Wohlfarth
    Boiler Lessons
    PC7060